Circuit arrangements of this kind are used, for example, in automotive electronics where the operation of fast-switching, inductive loads by means of MOS power transistors is increasingly required. Typical applications include the control of electromagnetic injection valves for diesel or gasoline high-pressure direct injection systems, 3-phase frequency converters for operating electric motors/generators with electronic commutation, bidirectional DC/DC converters or electromagnetic valve operation.
In these applications, MOS power transistors are often used as switches. Depending on the doping of the transistor substrate and of its channels, a distinction is drawn between Nand P-channel transistors, an N-channel transistor having n-doped regions in a p-doped substrate. In terms of circuit technology, it is particularly advantageous to implement so-called low-side switches, i.e. those which are essentially assigned to a load on the ground side, using N-channel transistors, and to implement so-called high-side switches, i.e. those which are essentially assigned to a load on the positive potential side, using P-channel transistors. This makes the required circuits particularly simple. However, for technological reasons, P-channel transistors have approximately twice the saturation resistance of an otherwise comparable N-channel transistor. In order therefore to implement a P-channel transistor with a predefined saturation resistance, approximately twice the chip area is required compared to a corresponding N-channel transistor. The associated costs are broadly proportional to the chip area required.
Attempts are therefore being made to implement even high-side switches with N-channel transistors, the main problem being that an N-channel transistor only becomes conducting between drain and source in the event of a positive gate voltage. In the case of a high-side switch, however, positive means higher than the positive potential at which conduction is to take place. Therefore, if e.g. in the case of controlling an injection valve the latter's coil is connected to the +48 V supply voltage of the vehicle electrical system, the gate voltage of an N-channel transistor used as a switch must be higher than this supply voltage. Various principles for solving this problem are known.
One possibility is the so-called charge pump whereby a small alternating voltage, preferably a square-wave control voltage, is clamped after capacitive decoupling with respect to the supply voltage, then rectified in a rectifier cascade comprising diodes and parallel capacitors and amplified. This type of voltage multiplier will be known to a person skilled in the art. A particularly favorable embodiment for controlling high-side switches has been realized e.g. in Infineon's BTS410 intelligent high-side switch. Although this principle enables high voltages to be generated in a simple and cost-effective manner, the current handling capacity of a voltage source produced in this way using a charge pump is mostly low. However, because of the gate capacitance, it is this high current flow to the gate electrode that is essential in order to enable fast switching of the MOS power transistor. The charge pump principle is therefore suitable primarily for slow-switching applications.
The problem can be solved in a basically relatively simple way by connecting an additional auxiliary voltage source whose negative terminal is applied to the positive potential of the supply voltage. This can be implemented, for example, by means of a DC/DC boost converter or in some other manner that will be familiar to a person skilled in the art. This high potential is then applied to the gate electrode via suitable switching means including e.g. a pnp switching-means transistor whose emitter is at the potential of the raised auxiliary voltage source and whose collector is connected to the gate electrode of the MOS power transistor, the base of the switching-means transistor being suitably driven by a control voltage. This circuit is suitable for delivering high currents, thereby enabling fast switching of the MOS power transistor. Because of the auxiliary voltage source required, which is raised above the supply potential and can therefore only be used for the explained purpose, the circuit described is expensive and only becomes economically viable when used for a plurality of high-side switches. Moreover, in order to also guarantee rapid turn-off, i.e. to ensure rapid draining away of the charge stored by the gate capacitor, additional circuitry must be operated. FIG. 2 shows a block diagram.
Another known arrangement for generating the high gate voltage of a high-side switch implemented as an N-channel transistor is the so-called bootstrap circuit whereby a low-voltage source, i.e. in this context a voltage source whose output voltage is much lower than the positive supply voltage, e.g. a 12 V source, which is generally present in a motor vehicle in addition to the 48 V supply voltage for supplying the general drive and control electronics, is coupled via a diode into a network having at least one switching-means transistor whose collector current can flow to the gate electrode of the MOS power transistor. A capacitor is provided in parallel therewith and connected to the source output of the MOS power transistor. In the off state, it basically charges up to the voltage of the low voltage source. If the switching-means transistor is then driven so that it connects the gate electrode to the low voltage source, the MOS power transistor begins to conduct, its source potential beginning to rise in the direction of the supply voltage. However, as the source electrode is connected to the negative pole of the capacitor, its voltage is also raised. As soon as it exceeds the voltage of the low voltage source, its charge flows via the switching-means transistor to the gate electrode, as the diode prevents it from flowing back into the low voltage source. In this way source and gate potential increase equally, their difference being approximately maintained. The MOS power transistor can therefore turn fully on. The capacitor can deliver a high current for a short period, thereby ensuring fast switching. The disadvantage, however, is that the current decays relatively quickly even with a high-value capacitor, so that lasting turn-on of the MOS power transistor is not possible with this circuit. There exist integrated driver circuits for operating high-side switches which are suitable for bootstrap supply, e.g. IC IR2122(S) from International Rectifier. FIG. 3 shows a block diagram.
The main disadvantage of the circuit arrangements according to the prior art is that the switching edges, i.e. basically the switching rate, is only adjustable to a very limited extent. This is disadvantageous because, for fast switching processes, a balanced compromise must always be found between excessively slow switching on the one hand, resulting in heating of the transistors and therefore increased switching losses, and excessively rapid switching on the other, resulting in an increased electromagnetic interference level. Typically the room for maneuver between these two contrary requirements is very small, so that a very precise examination of the switching times would be desirable.